The enzymic mechanisms that regulate the activities of the glycogen chain-lengthening (synthase) and chain-shortening (phosphorylase) enzymes of liver are under investigation, with special reference to the protein phosphatases, and the question whether there are separate phosphatases for each phosphoenzyme or whether one phosphatase acts on two or more phosphoenzyme substrates. Present indications are that liver phosphosynthase and phosphophosphorylase are common substrates for a single phosphoprotein phosphatase. The mechanisms of biosynthesis of amylopectin and glycogen are being studied using the hypothesis that branching occurs by the association of two chains to form a double helix, and that the double helix is the substrate for the branching enzyme. This hypothesis has important implications in regard to the structure of amylopectin and glycogen.